Whether or not someone can 'beat' me without the stats in an RP purely depends on how they RP. If their RP tells me they clearly took the time to learn the game, learn the settings and put effort into how they are interacting so it's involving people and not all one sided, I am more than happy to completely ignore stats and let them win (or lose, as the case might be).
I have often 'beaten' highly leveled characters from a no-level alt and never once had any complaints about it due to how I've gone about the RP. On the other hand, if I walked up and said "I stick a dagger in your back and instantly kill you because I am the best dagger fighter ever!!" I'd fully expect to be laughed out of the game, because that just doesn't cut it.
In short, if playing a no-level bad guy, give RP value. Make the experience of losing to you interesting and enjoyable and don't rush to beat 'the good guys.' Plus a quick /tell before you begin generally goes a really long way.
The balance between how much stats and skill levels should or shouldn't be involved in RP is always going to be a tricky one and both sides have strong points in their favour, but when it comes down to it, if both players can't enjoy themselves there is no real point in the RP anyway, so should work together.
I just try to go on 'experience' of the game rather than just levels. Levels are one way to show experience of the game (though not necessarily experience of RPing well). How someone goes about the RP can also show it, and to me that's of equal, if not more, value than the levels themselves.